The goals of this research project are to study: 1) the ontogeny of the autonomic control of the cardiovascular functions from the fetal to the neonatal stages of development and compare the findings to those of the adult sheep. Emphasis will be placed on the development of the neurohumoral control of heart rate, vascular pressures, cardiac output and regional blood flows in the resting normal condition; 2) the changes that occur in the neurohumoral control during various forms of stress such as hypovolemia, hypertension and hypoxia; 3) the mechanisms by which estrogenic hormones produce uterine vasodilatation and the impact on the fetus. In the majority of instances, the approach to be used in this work is the chronically-instrumented, unanesthetized animal. Acute experiments will be employed only when it would be impossible to perform the investigative procedures in the chronically-instrumented animal. The In-Vivo studies will be supplemented by In Vitro studies of the heart and blood vessels obtained from fetuses and neonates at different stages of development.